The invention relates to belted tire construction and more particularly to a construction including a folded belt containing glass or other reinforcement fibers.
The use of glass fibers in various forms as carcass and tread reinforcement for pneumatic tires is well known and has a number of unique advantages not afforded by other materials. See, for example, U.S. Pat. Nos. 2,184,326, 2,884,040, 2,894,555, 3,315,722 and 3,523,472. In tires incorporating tread reinforcing belts, various belt folding constructions have been suggested, particularly in the case of steel wire reinforced belts. See U.S. Pat. Nos. 3,404,721 and 3,623,529.
Tire belt folding configurations such as those disclosed in the above '721 and '529 patents are designed to overcome a tire wear problem known as belt edge or breaker edge separation. Generally, tread reinforcing belts, or tread plies, are positioned within the crown of a tire between carcass and tread as generally flat, annular bands which extend circumjacent the carcass. There may be one, two or more of these annular bands positioned generally parallel and concentrically adjacent one another. The belts almost universally contain parallel cords of organic, steel or glass materials oriented on a bias angle from the circumferential center line of the tire, with adjacent belts generally being oriented at opposed bias angles.
The problem of belt edge separation generally arises from road stresses acting in the shoulder areas of the tire where the edges of the belts lie. These extreme shoulder area stresses are caused primarily by the rotation of the load-bearing tire into its contact patch with the pavement. The continual changing in shape of the tire's cross section causes the stresses, concentrating them in the shoulder areas. The belt edges within the shoulder areas generally contain exposed ends of the bias oriented cords. Thus, the interface between the cord and the surrounding rubber or elastomeric material is often the weakest bond area between the belts and the tire crown within which they are encapsulated. The shoulder area stresses therefore tend to cause a separation of the cord end from the adjacent rubber, thereby resulting in a gradual ply separation which can be aggravated by high speed conditions.
Belts having folds, rather than raw edges, in the shoulder area of the tire have been proposed as a solution to the above problems. The folds not only remove the exposed-cord edges of the belt from the high-stress shoulder areas, but also act to somewhat stiffen the tire shoulders so that less movement is encountered in these shoulders and stress on the belts in this area is effectively reduced. Thus, the effect of the folded belts is to stiffen the lateral edges of the tread and to some degree interlock them so that the crown reinforcement as a whole acts more uniformly in avoiding shifting of the reinforcement cords, particularly the ends of the cords.
However, the use of such folded belts has met with some problems. Regardless of the type of fiber used in the cords of the plies, a sharply folded belt ordinarily does not impart the needed stiffness to the tire shoulder and the wear characteristics of the tire are thus not what they could be. Moreover, when metal or glass fiber belt cords are used, it has been suggested that the cords tend to break along the sharp fold lines due to bending stresses encountered when the belt edges are folded in the manufacturing process and repeated during tire usage. The above-mentioned U.S. Pat. No. 3,404,721 proposes a solution to this latter problem in the case of metal belt reinforcement by using multiple belts in which the fold of one encompasses the outer edge of another, and the folded belt employs metal cords having a lower modulus of elasticity than those of the flat belt. This, of course, requires relatively complex construction in a belted tire.